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: in the flow context

Forbid ':' for plain scalars in the flow context and use it to separate keys from values.
his syntax is more Python-compatible. For instance, it allows

{1:2,2:3,3:5,4:7,5:11,6:13,7:17,8:23} # prime numbers

Allow ':' for plain scalars in the flow context provided it's not followed by a whitespace.
Require ': ' to separate keys from values.
This will allow unquoted time values and URLs in the flow context. For instance,

[12:45, http://pyyaml.org/] # Time and URL

What do you think? Please leave an excerpt of your document which produced an error here,
in the wiki page.

It would make sense to require ': ' when parsing a value - if you see a:b when parsing a value, you should treat that as the string "a:b". When parsing a key-value in a mapping, it should be possible to allow either url:http://pyyaml.org/ (no space) or url: http://pyyaml.org/ as you know from context that you're expecting a : to terminate a key. Of course, colons in keys are pathological and should not be tolerated ;-)

Solution #1 may be natural for Python but not for other languages. Since YAML should stay language-neutral solution #2 would meet much more expectations.